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    <title>Document</title>
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    <script>
        const PI = 3.14; // 定义常量PI, 必须在声明时初始化, 且不能修改
        document.write(PI);
        PI = 3.14159; // 尝试修改常量PI
        document.write(PI); // 输出修改后的PI
    </script>
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